This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Bacterial multicomponent monooxygenases (BMMs) such as methane monooxygenase (MMO), toluene/o-xylene monooxygenase (ToMO), and phenol hydroxylase (PH) catalyze the regio- and enantioselective oxidation of saturated and aromatic hydrocarbons to yield alcohols, phenols, catechols, and epoxides. Regulatory and reductase protein components are required to effect catalysis in BMM hydroxylases, which contain carboxylate-bridged non-heme diiron active sites at which dioxygen-activated intermediates form and oxidize the hydrocarbon substrates. Crystallographic characterization of xenon-pressurized, substrate/product-soaked, and mutant forms of the ToMO hydroxylase and PH hydroxylase and hydroxylase-regulatory complex are sought to probe substrate access pathways, proton and electron transfer mechanisms, and structure-activity relationships in BMMs and diiron carboxylate protein in general. These data will complement biochemical findings on the proteins of interest and provide additional information for comparison with the MMO system in order to elucidate the mechanisms of biological hydrocarbon oxidation.